Cultured murin embryolines (SW 3T3, Balb3T3, M2, C3H10T1/2) exhibit growth control. The cells stop dividing when confluent, although medium from growth-arrested cells can support the division of pre-confluent cells. A variety of agents of diverse chemical structures (e.g. insulin, fibroblast growth factor, cortisol, epidermal growth factor, phorbol myristate acetate, prostaglandin F2 alpha) can initiate cell division. Two alternative models for initiation of proliferation exist. The first (our "ubiquitous pathway model") postulates that all mitogenic agents, after interaction with their specific receptors, trigger a common pathway. The second ("unique pathway") model postulates unique biochemical steps for individual mitogens (or at least for classes of mitogens) in the initiation of proliferation. We plan to select non-proliferative clones for specific mitogens. In these two models the phenotypes of non-proliferatvie variants will differ. In the former case the only allowable alteration is the loss of the mitogen receptor; all other alterations in the proliferative response will be lethal. In the latter case receptor positive non-proliferative variants with defects in the unique mitogenic pathway should occur. We should be able to distinguish between these alternatives by isolating and characterizeing a number of variants (including single mitogen, double mitogen and temperature-sensitive clones) to a variety of chemically distinct mitogens. We have already selected variant clones of 3T3 lacking a proliferative response to Epidermal Growth Factor. Alternative selection procedures and characterization of variants to other mitogens are described. Variants missing receptors (present in both the "unique" aand "ubiquitous" proliferation models) are appropriate controls for binding studies, receptor characterization, receptor-induced membrane alterations, etc. Receptor positive non-proliferative variants can be used to distinguish between events in the "pleiotypic response" which are necessary vs. those which are correlative.